PHP加密解密明文相同每次密文不一样非对称加解密

//加密：
	//密钥 把数据装入二进制（更安全）
	$key = pack('H*', "bcb04b7e103a0cd8b54763051cef08bc55abe029fdebae5e1d417e2ffb2a00a3");
	echo 'key::'.$key.'

';
	//看下二进制数据长度
    $key_size =  strlen($key);
    echo "Key size: " . $key_size . "

\n";
    
    $plaintext = "This string was AES-256 / CBC / ZeroBytePadding encrypted.";

    # create a random IV to use with CBC encoding
    $iv_size = mcrypt_get_iv_size(MCRYPT_RIJNDAEL_128, MCRYPT_MODE_CBC);
    $iv = mcrypt_create_iv($iv_size, MCRYPT_RAND);
    
    $ciphertext = mcrypt_encrypt(MCRYPT_RIJNDAEL_128, $key, $plaintext, MCRYPT_MODE_CBC, $iv);

    $ciphertext = $iv . $ciphertext;
    
    $ciphertext_base64 = base64_encode($ciphertext);
	//输出密文（每次都不一样，更安全）
    echo  '

jia mi:::'.$ciphertext_base64 . "\n";

//解密：
	$ciphertext_dec = base64_decode($ciphertext_base64);    
    $iv_dec = substr($ciphertext_dec, 0, $iv_size);    
    $ciphertext_dec = substr($ciphertext_dec, $iv_size);
    $plaintext_dec = mcrypt_decrypt(MCRYPT_RIJNDAEL_128, $key, $ciphertext_dec, MCRYPT_MODE_CBC, $iv_dec);
    
    echo  '

jie mi:::'.$plaintext_dec . "\n";
	exit;